Cats and Rats
In the Clover
The story of the anticoagulant-cum–rat poison begins with an agricultural strategy of the early 1900s, the introduction of sweet clovers in the North American prairies. In 1924 ranchers in Alberta, Canada, observed that cattle feeding on moldy sweet clover were dying from a hemorrhagic disorder; the active agent was found to be dihydroxycoumarin (coumarin itself is shown below). This molecule was synthesized within a year and quickly found use as an anticoagulant. Meanwhile warfarin, the derivative of coumarin, was developed by University of Wisconsin biochemists Mark A. Stahmann and Karl P. Link, and used as a rodenticide. It took 10 years for it to enter clinical practice as a blood-thinner.
Coumarin smells good—the parent molecule is described in the Merck Index as having a "pleasant, fragrant odor, resembling that of vanilla beans...." There is even a story of it being used to enhance vanilla, a practice banned, for obvious reasons, some time ago. Nevertheless, some tobacco-leaf preparations were soaked or "cased" in coumarin to add aroma.
So what's in it for the clover? Defense, again. And sweet clovers aren't the only greens using coumarins. Many plants make use of furocoumarins, in which a coumarin is bonded to a furan ring, as protective substances. May Berenbaum of the University of Illinois has shown that such compounds are quite effective in protecting parsnips from insect predators (except for the ones that have evolved countermeasures). A further quirk is that furocoumarins are photodynamic—their toxic effects in the animal are triggered by sunlight. Humans are not immune: There's a report in the literature on "celery phytophotodermatitis in a chef." And an unnamed young woman made the medical literature with a case of "Club Med photodermatitis" following a poolside game of rolling limes between her thighs. Watch out for veggies.